The cost of survival: mutation in a barley Strigolactone repressor <i>HvD53A</i> impairs photosynthesis but increases drought tolerance

Strigolactones (SLs) are a class of plant hormones that play a crucial role in shaping plant architecture, significantly influencing plant adaptation to harsh environmental conditions. In this study, we examined the effects of a mutation in a component of the barley SL signalling pathway, the SL repressor HvDWARF53A, on plant growth and drought tolerance. We compared the results with those of a previously described barley mutant, which is highly tillered and drought-sensitive, carrying a mutation in the SL receptor gene HvDWARF14. The two mutants, hvd14.d and hvd53a.f, displayed contrasting phenotypes, including differences in plant height, tillering, and drought sensitivity. Under control conditions, ultrastructural analysis of hvd53a.f revealed smaller chloroplasts and fewer grana stacks, which may account for its reduced photosynthetic efficiency. Conversely, transcriptomic analysis linked the differentially expressed genes in hvd53a.f to antioxidation and stress responses, suggesting a potentially enhanced capacity to cope with drought. Further analysis revealed a strong connection between the SL signalling pathway and circadian clock components. Among these, CIRCADIAN CLOCK ASSOCIATED 1 emerged as a potential SL-responsive transcription factor (TF), possibly playing a key role in regulating tillering. Under drought conditions, hvd53a.f exhibited enhanced tolerance, as evidenced by higher relative water content, reduced chlorophyll degradation, and stable, albeit reduced, photosynthetic performance. Here, we identified the SL-related TF JUNGBRUNNEN 1 as a potential regulator of genes involved in water deficit response and antioxidation processes. Overall, the hvd53a.f mutation enhances drought tolerance while maintaining low, stable photosynthesis, highlighting HvD53A as a central node connecting SL signalling to stress resilience.